1. Field of the Invention
This invention relates to the production of radiation curable microcapsular coating compositions. In particular, it relates to the production of microcapsules containing a hydrophilic core by reaction of two wall-forming materials in a hydrophobic liquid wherein the hydrophobic liquid is a radiation curable organic liquid. In one embodiment of this invention, the encapsulated hydrophilic liquid contains a chromogenic material soluble in the encapsulated hydrophilic liquid. A dispersion of these microcapsules can be coated on a substrate and cured by radiation to give a pressure-sensitive carbonless copy sheet having a transfer coating.
2. Prior Art
The production of microcapsules containing an encapsulated oily (hydrophobic) liquid wherein the microcapsule walls are produced by reaction of polyisocyanate and a second wall-forming material is described in U.S. Pat. No. 3,796,669 to Kiritani et al. Both the polyisocyanate wall-forming material and the second wall-forming material are mixed with the oily liquid. The mixed oily liquid is dispersed into an aqueous continuous phase to form a dispersion of oil drops and the temperature is raised to initiate the reaction on the surface of the oil drops thus encapsulating the oil drops with the reaction product of the polyisocyanate and second wall-forming material. A catalyst for the reaction may also be added to the oily liquid.
Carbonless copy paper, briefly stated, is a standard type of paper wherein during manufacture the backside of the paper substrate is coated with what is referred to as a CB or transfer coating, the CB coating containing one or more chromogenic materials, generally in capsular form. At the same time the front side of the paper substrate is coated during manufacture with what is referred to as a CF coating, which contains one or more chromogenic materials capable of producing a color with the encapsulated CB chromogenic material. Both the chromogenic materials remain in the coatings on the respective back and front surfaces of the paper in substantially colorless form. This is true until the CB and CF coatings are brought into overlying relationship and sufficient pressure, as by a typewriter, is applied to rupture the CB coating to release the encapsulated chromogenic material. At this time the chromogenic material contacts the CF coating and reacts with the chromogenic material therein to form a colored image. Carbonless copy paper has proved to be an exceptionally valuable image transfer media for a variety of reasons, only one of which is the fact that until a CB coating is placed next to a CF coating both the CB and CF coatings are in an inactive state as the coreactive elements are not in contact with one another until pressure is applied. Patents relating to carbonless copy paper products are:
U.s. pat. No. 2,712,507 (1955) to Green PA1 U.s. pat. No. 2,730,456 (1956) to Green et al. PA1 U.s. pat. No. 3,455,721 (1969) to Phillips et al. PA1 U.s. pat. No. 3,466,184 (1969) to Bowler et al. PA1 U.s. pat. No. 3,672,935 (1972) to Miller et al. PA1 U.s. pat. No. 3,551,235 (1970) to Bassemir et al. PA1 U.s. pat. No. 3,551,246 (1970) to Bassemir et al. PA1 U.s. pat. No. 3,551,311 (1970) to Nass et al. PA1 U.s. pat. No. 3,558,387 (1971) to Bassemir et al. PA1 U.s. pat. No. 3,661,614 (1972) to Bassemir et al. PA1 U.s. pat. No. 3,720,534 (1973) to Macaulay et al. PA1 U.s. pat. No. 3,754,966 (1973) to Newman et al. PA1 U.s. pat. No. 3,772,062 (1973) to Shur et al. PA1 U.s. pat. No. 3,772,171 (1973) to Savageau et al. PA1 U.s. pat. No. 3,801,329 (1974) to Sandner et al. PA1 U.s. pat. No. 3,819,496 (1974) to Roskott et al. PA1 U.s. pat. No. 3,847,768 (1974) to Kagiya et al. PA1 U.s. pat. No. 3,847,769 (1974) to Garratt et al.
A disadvantage of coated paper products such as carbonless transfer paper stems from the necessity of applying a liquid coating composition containing the color forming ingredients during the manufacturing process. In the application of such coatings, volatile organic solvents are sometimes used which then in turn requires evaporation of excess solvent to dry the coating thus producing volatile solvent vapors. An alternate method of coating involves the application of the color forming ingredients in an aqueous slurry, again requiring removal of excess water by drying. Both methods suffer from serious disadvantages. In particular, the organic solvent coating method necessarily involves the production of generally volatile solvent vapors, creating both a health and a fire hazard in the surrounding environment. When using an aqueous solvent system the water must be evaporated which involves the expenditure of significant amounts of energy. Further, the necessity of a drying step requires the use of complex and expensive apparatus to continuously dry a substrate which has been coated with an aqueous coating compound. A separate but related problem involves the disposal of polluted water. The application of heat not only is expensive, making the total paper manufacturing operation less cost effective, but also is potentially damaging to the chromogenic materials which are generally coated onto the paper substrate during manufacture. High degrees of temperature in the drying step require specific formulation of coating compositions which permit the use of excess heat. The problems encountered in the actual coating step are generally attributable to the necessity for a heated drying step following the coating operation.
The novel process and liquid coating composition of this invention are superior to those used in the prior art microcapsular coating of substrates in that they do not need an organic solvent or water in their coating composition, thus avoiding the disadvantages associated with solvent removal during drying. The liquid radiation curable substance is a solvent for the wall-forming material in the hydrophobic liquid. The liquid radiation curable substance cures by radiation to give a tack-free film containing microcapsules. The cured film acts as a binder to adhere the microcapsules to the substrate.
In general, patents concerned with the production and application of liquid resin compositions containing no volatile solvent which are subsequently cured by radiation to a solid film are:
These compositions generally also contain a pigment or dye. Such resin compositions are useful for protective coatings and fast drying inks. U.S. Pat. No. 3,754,966 describes the production of an ink releasing dry transfer element which can be used as a carbon paper or typewriter ribbon. It is significant to note here that the particular radiation cured coating must be compatible with the reaction of CB and CF chromogenic materials to form a color. Such color forming reactions are generally of a sensitive or delicate nature and are not generally compatible with the compositions found in the prior art.
The novel liquid coating compositions of this invention contain microcapsules having an aqueous core liquid. The microcapsule walls are produced by a reaction of two wall-forming materials in a radiation curable hydrophobic liquid. Prior to the discovery of this invention, it was not known that such microcapsules could be produced in situ in radiation curable liquid compositions. For purposes of this disclosure, a tack-free film is one which will separate cleanly from a cotton ball lightly pressed against the film. The cotton fibers will not adhere to the film surface.
An especially preferred application of the process of this invention is in the continuous production of a manifold carbonless form. The continuous production of a manifold paper product would require simultaneous coating, simultaneous drying, simultaneous printing, and simultaneous collating and finishing of a plurality of paper substrates. Thus, Busch in Canadian Pat. No. 945,443 indicates that in order to do so there would be a minimum wetting of the paper web by water during application of the CB emulsion coat. For that purpose a high solids content emulsion is used and special driers are described in Busch. However, because of the complexities of the drying step, this process has not been commercially possible to date. More particularly, the drying step involving solvent evaporation and/or water evaporation and the input of heat does not permit the simultaneous or continuous manufacture of manifold forms. In addition to the drying step which prevents continuous manifold form production the necessity for the application of heat for solvent evaporation is a serious disadvantage since aqueous coatings require that special grades of generally more expensive paper be employed and even these often result in buckling, distortion or warping of the paper by the water present in the coating. Additionally, aqueous coatings are generally not suitable for spot application or application to limited areas of one side of a sheet of paper. They are generally suitable only for application to the entire surface area of a sheet to produce a continuous coating.
Another problem which has been commonly encountered in attempts to continuously manufacture manifold forms has been the fact that a paper manufacturer must design paper from a strength and durability standpoint to be adequate for use in large variety of printing and finishing machines. This requires a paper manufacturer to evaluate the coating apparatus of the forms manufacturers he supplies in order that the paper can be designed to accommodate the apparatus and process designed exhibiting the most demanding conditions. Because of this, a higher long wood fiber to short wood fiber ratio must be used by the paper manufacturer than is necessary for most coating, printing or finishing machines in order to achieve a proper high level of strength in his finished paper product. This makes the final sheet product more expensive as the long fiber is generally more expensive than a short fiber. In essence, the separation of paper manufacturer from forms manufacturer, which is now common, requires that the paper manufacturer overdesign his final product for a variety of machines, instead of specifically designing the paper product for known machine conditions.
By combining the manufacturing, printing and finishing operations into a single on-line system a number of advantages are achieved. First, the paper can be made using ground wood and a lower long fiber to short fiber ratio as was developed supra. This is a cost and potentially a quality improvement in the final paper product. A second advantage which can be derived from a combination of manufacturing, printing and finishing is that waste or re-cycled paper hereinafter sometimes referred to as "broke" can be used in the manufacture of the paper since the quality of the paper is not of an overdesigned high standard. Third and most importantly, several steps in the normal process of the manufacture of forms can be completely eliminated. Specifically, drying steps can be eliminated by using a non-aqueous, solvent-free coating system and in addition, the warehousing and shipping steps can be avoided, thus resulting in a more cost efficient product.
Additionally, by using appropriate coating methods, namely radiation curable coating compositions and methods, and by combining the necessary manufacturing and printing steps, spot printing and spot coating can be realized. Both of these represent a significant cost savings but nevertheless one which is not generally available when aqueous coatings are used or where the manufacture, printing and finishing of paper are performed as separate functions. An additional advantage of the use of radiation curable coating compositions and the combination of paper manufacturer, printer and finisher is that when the option of printing followed by coating is available significant cost advantages occur.